I think there should be a poop meter in the game. This meter slowly goes up on its own, especially faster when you eat healing fruit or drink blood boxes/blood kegs. If you stay in combat with a large amount of enemies for a long time, the poop meter goes up. When you reach certain strong setpieces (Like the huge amounts of pupae in the Infernal Falls or the huge amounts of manta on Bitter Bridge), the poop meter suddenly shoots up. When the poop meter is full, you get +10 Shitgun ammo.

Gameite; "Shit sounds like a fable to me", till he jumps out the fuckin toilet when you're takin a pee.